Ceramic reinforced polymer matrix composites have become well known in recent years and are now generally regarded as being superior to the silicate cements and direct filling resins which were previously used as dental restorative materials. Such composites normally consist of a methacrylate-based system in which a silica glass filler is covalently bonded to a resin matrix, or to a coupling agent which is covalently bonded to both. The finely-divided filler material reinforces the resin matrix and, when used to the 80 percent level, greatly reduces polymerization shrinkage and thermal expansion in comparison with the previously-used direct filling resins. In terms of stability, insolubility, strength, and general performance, the ceramic reinforced polymer matrix composites have overcome many of the objections raised in connection with earlier dental restorative materials.
The aesthetic superiority of modern dental composites is accompanied by at least one disadvantage, however. Not only are such dental restorations difficult or impossible to distinguish from natural tooth surfaces by normal visual inspection, they are also hard to distinguish by x-ray analysis. Ideally, a superior dental restoration should be indistinguishable from the natural tooth under ordinary light but should stand out clearly in an x-ray photograph so that a dentist can easily distinguish the limits of the restoration in searching out any carious dentin that may underlie the filling.
Efforts to provide radio-opaque dental composites have met with only very limited success. Glass fillers formulated to contain enough barium to achieve appreciable radio-opacity are otherwise unsatisfactory because of the solubility of the glass at high concentrations of barium. Since the solubility decreases at lower concentrations, it has been found possible to prepare barium-containing glasses in which no appreciable leaching of the barium will occur; however, in such a case the barium concentration is so low that radio-opacity is virtually lacking (see photograph).
The solubility of barium-containing glass fillers has led some workers to incorporate barium carbonate in the filler composition. While barium carbonate has the advantage of being relatively insoluble, it has the offsetting disadvantages of being relatively soft and, in general, of weakening the composite. In its carbonate form, the barium does not become part of the reinforcing filler but simply serves as a radio-opaque additive. If the barium carbonate is included in sufficient amounts to produce satisfactory radio-opacity, then an unacceptable weakening of the composite may result, whereas if the amount is kept low enough to maintain the strength and durability of the product, radio-opacity is marginal.